day3: first part

Author: fxnn

day3/example.txt

@@ -0,0 +1 @@
+xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))

day3/main.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"github.com/fxnn/adventofcode2024/util"
+	"os"
+	"regexp"
+)
+
+func main() {
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Split(bufio.ScanLines)
+
+	var text string
+	for scanner.Scan() {
+		text += scanner.Text()
+	}
+
+	pattern := regexp.MustCompile(`mul\((\d+),(\d+)\)`)
+	result := 0
+	for _, match := range pattern.FindAllStringSubmatch(text, -1) {
+		n1 := util.Atoi(match[1])
+		n2 := util.Atoi(match[2])
+		result += n1 * n2
+	}
+
+	fmt.Printf("result: %d\n", result)
+}

util/util.go

@@ -1,6 +1,21 @@
 package util
 
-import "fmt"
+import (
+	"fmt"
+	"os"
+	"strconv"
+)
+
+// Atoi converts string to integer, and exist immediately upon error
+func Atoi(s string) int {
+	if i, err := strconv.Atoi(s); err != nil {
+		fmt.Fprintf(os.Stderr, "Error in Atoi(%s): %s\n", s, err)
+		os.Exit(1)
+		return 0
+	} else {
+		return i
+	}
+}
 
 // Abs calculates the absolute value of an integer
 func Abs(x int) int {